1. Field of the Invention
Exemplary aspects of the present invention generally relate to an optical scanning device including a cantilever, and an image forming apparatus or an image projection device employing the optical scanning device.
2. Description of the Background Art
Conventionally, an optical scanning device employed in image projection devices and image forming apparatuses such as digital copiers and laser printers, utilizes a deflector such as a polygon mirror or a galvanometer mirror to scan a light beam projected from a light source. In order to achieve a high-resolution image and high-speed operation, typically, high operational speed and a relatively large swing angle of those mirrors are of critical importance.
In view of the above, much research has been conducted on an optical deflector using silicon micromachining in recent years. For example, Japanese Patent Nos. 2924200 and 3011144 propose an optical deflector in which an oscillation mirror and a torsion beam supporting the mirror constitute a single integrated unit on a silicon (Si) substrate.
Although an optical scanning device using such an oscillation mirror is advantageous in that it consumes less power when compared with the optical scanning device using the polygon mirror, a large swing angle at high speed is required of the mirror. In order to obtain a large swing angle of the mirror, it is necessary to swing widely the cantilever that provides torsional torque to the torsion beam. As a result, the cantilever may break or be damaged when swung widely.
Etching is one example of a method for fabricating a cantilever having good mechanical strength and thus difficult to break. In this method, the cantilever is deliberately made to have a thickness distribution that is uneven in a depth direction. That is, a fixed end of the cantilever is made thicker, thereby reducing concentration of stress and thus preventing destruction of the cantilever. Such a processing method is known to be effective in the field of Micro Electro Mechanical System (MEMS) when fabricating the cantilever on the silicon substrate.
However, there is a drawback to this method. When forming an optical scanning device using a single sheet of the silicon substrate, the amount of etching needed for fabrication of the torsion beam and the cantilever increases, resulting in irregular thickness of the substrate due to etching. In particular, the irregular thickness of the torsion beam generates irregular vibration, thus resulting in irregular resonance frequency. When the irregularity of the resonance frequency becomes significant, it complicates drive control, thereby complicating design of the drive circuit and increasing its cost.
In view of the above, there is demand for a cantilever that is physically strong to withstand torsional rotation while providing a large swing angle and suppressing irregular resonance frequency.